It is known that numerous systems for the generation of energy are based upon a pair of turbo-units operating in tandem-compound configuration, in which each turbo-unit includes an electric generator driven on the same shaft by a steam turbine supplied by an oil-burning or coal-burning boiler of its own, with subcritical steam conditions both on superheated (SH) steam for high-pressure (HP) admission and on re-superheated (RH) steam for medium-pressure (MP) admission. Steam turbines are generally of the two-body type (a combined HP-MP section and a low-pressure (LP) section).
The above type of systems presents a relatively low efficiency, so that the tendency is to convert such a system into a system of approximately the same power, which presents supercritical and/or ultrasupercritical conditions at HP and MP admission (in order to increase the efficiency) and is based upon a single boiler, which supplies both of the pre-existing turbines, reconfigured into a cross-compound configuration.
For the above purpose, the known art envisages, in addition to replacing the two boilers with a new boiler of approximately twice the horsepower, replacement of the two steam turbines or of at least the two combined HP-MP sections with as many new sections in order to meet up to the higher design conditions (pressure and temperature of the steam at admission to the HP and MP sections), for which the materials and the original design of the pre-existing turbines are no longer adequate.
Said solution is not, however, free from drawbacks. In addition to being costly, the new steam-turbine sections present, in fact, levels of efficiency that are panalized, as compared to the new supercritical or ultrasupercritical steam conditions, by the number of stages limited by the encumbrance of the existing foundation.
With a single boiler, which supplies the two turbo-units, the system of regulation and the running of the turbo-units themselves together represents, then, an element of greater complexity. It is, in fact, necessary to provide manifolds for the SH and RH steam, from which both of the turbo-units are to be supplied in branched fashion, and it is problematical to adapt the steam conditions to the requirements of the two turbo-units (which may be different from one another, for example, with one unit in use and the other unit in its starting up stage after a stoppage). For instance, it is necessary to double, among other things, the total number of the main valves and the number of the regulation and protection systems.
Systems in cross-compound configuration with different lay-outs are also known to the art, for example from U.S. Pat. No. 4,316,362 and JPA59-60008. The configurations known from said documents are, however, suitable only for newly-devised systems, and not for reconfiguration of pre-existing systems. Furthermore, the systems known from said documents present complex lay-outs.